Silicon ceramics such as silicon nitride and silicon carbide are the most promising materials for exacting structural ceramic applications. Their specific properties as compared with traditional metal materials include a high strength and resistance to corrosion even at high temperatures, a light weight, and resistance to wear. Compared with other ceramic materials, silicon nitride and silicon carbide also have good thermal shock resistance.
The most important areas of use of silicon ceramic materials at present include small machine components such as cutting bits, machine tools, seals, and bearings. It is forecast that, as manufacturing techniques develop, largescale use will be attained in the 1990s, at which time more and more ceramic components will be used, for example, in automobile engines. It is a prerequisite for this technological leap that ceramic raw materials of a sufficiently high standard can be produced at a competitive price.
At present, silicon nitride is produced commercially mainly by treating silicon powder at a high temperature (1200.degree.-1450.degree. C.) with nitrogen or ammonia (for example F. L. Riley, Progress in Nitrogen Ceramics, Nato ASI Series No. 65, 1983, pp. 121-134). The slow diffusion of nitrogen in silicon makes the process slow. The particle size of the product is usually as such too large for exacting applications, and therefore the commercial product is usually ground. Grinding for its part introduces into the powder non-desirable impurities, which also reduce the usability of the product for exacting applications.
Another prior-art method is to reduce natural quartz by means of carbon to silicon, which is then further immediately nitrided by means of ammonia or nitrogen (for example M. Mori et al., Progress in Nitrogen Ceramics, Nato ASI Series No. 65, 1983, pp. 149-156). The reaction temperature is 1200.degree.-1450.degree. C. The process involves the same problems as does direct nitriding of silicon.
Two types of processes based on the halides of silicon are known. As a result of a gas-phase reaction between silicon tetrachloride and ammonia, silicon nitride powder is deposited at approximately 900.degree. C. by a CVD-type process (for example P. E. D. Morgan, Production and formation of Si.sub.3 N.sub.4 from precursor materials, Franklin Institute Research Laboratories, 1974, Distributed by NTIS). The process is at the pilot stage, and its primary problem lies in the separation of the ammonium chloride produced as a byproduct.
Yamada et al. (Proc. Int. Symp. Ceramic Components for Engine, Japan, 1983, pp. 333-342) for their part describe a low-temperature process which is based on a reaction between silicon tetrachloride and liquid ammonia. Silicon imide Si(NH.sub.2).sub.2 is formed as an intermediate product, which is converted by calcination at approximately 900.degree. C. to a finely-divided silicon nitride powder. The process is in commercial use in Japan, and it is characterized by high production costs due to the slowness of the process and its numerous steps, the price of the product being therefore high considering large-scale use of the powder.
Processes for producing silicon nitride whiskers are described in, for example, U.S. Pat. Nos. 4,604,273 and 4,521,393. In both these processes, fibrous silicon nitride is formed in a reaction among silica, carbon and nitrogen in the presence of promoters. In the former process, metals such as chromium, nickel and magnesium are added as a promotor; in the latter, fluoride in the form of cryolite is added.
Silicon carbide has been produced traditionally by the carbothermal route by the Acheson process, in which quartz sand is reduced with carbon in an electric furnace. The product is impure considering fine-ceramic uses. By using pure raw materials it is possible to increase the purity of the product [for example JP-77117899 (1977), Toshiba Ceramics Co., Ltd., summarized in CA-88(1978)9157f and DE-2 848 377 (1978), Electroschmelzwerk Kempten], but the product must in any case be ground before the actual production of the ceramic. In carbothermal processes the production temperature of silicon carbide is usually 1500.degree.-2200.degree. C.
By gas-phase processes it is possible to produce a finely-divided powder without a grinding step which introduces impurities. The silicon and carbon sources which have been used include monosilane SiH.sub.4 together with a hydrocarbon, e.g. methane, and organochlorosilanes. In JP Patent 59102809 [Toshiba Ceramics Co., Ltd., summarized in CA 101(1984)156401w], an extra fine SiC powder was obtained by thermal decomposition of methyl trichlorosilane CH.sub.3 SiCl.sub.3 at 1550.degree.-2100.degree. C. The thermal decomposition of tetramethyl silane at 800.degree.-1400.degree. C. also produced silicon carbide (J. Less-Common Metals 68(1979), pp. 29-41]. Plasma has been used in a synthesis between monosilane and methane (for example JP-57175718 (1982), Hitachi, Ltd.). The problem of these processes as compared with the present process is their relatively expensive raw material which is difficult to obtain and which, together with the relatively low yield, makes the processes costly.
Other silicon sources which have been used for silicon carbide include silicon monoxide [DE-3 602 647 (1985), Toyota Motor Co., Ltd.] and silicon tetrafluoride [JP-58115016 (1981), Onoda Cement Co., Ltd., summarized in CA 99(1983)124972g]. Both of the processes have two steps. In the first-mentioned process, SiO was first produced by oxidizing a silicon powder, whereafter the SiO gas was fed into the synthesis together with methane. In the latter process, SiF.sub.4 was first reduced with sodium to silicon, which was thereafter mixed with a carbon component (vinyl chloride). The synthesis temperature was 1400.degree. C.
The use of silicon fluoride as the initial material for silicon nitride powder is known, for example, from Japanese Patent 59 174 506, according to which silicon tetrafluoride and ammonia are fed, under a lowered pressure, at a temperature of 900.degree.-1400.degree. C., through a carbon bed. The thermodynamic prerequisites and kinetics of this reaction are, however, disadvantageous and, furthermore, the separation of the product from the carbon bed is cumbersome.
Silicon difluoride SiF.sub.2 and its polymeric forms, having the chemical formula Si.sub.n F.sub.2n+2, where n is an integer which is .gtoreq.2, are prior known from the production of thin silicon nitride coatings. One possible initial material mixture is Si.sub.2 F.sub.6 +N.sub.2 +H.sub.2, which is described, for example in Japanese Patent 61 99 676 [Chemical Abstracts 105(1986)157791t] and in the publication of Fujita et al. in Japan. J. Appl. Physics 23(1984), pp. L268-L270. A silicon nitride film deposited at a temperature of 300.degree.-350.degree. C. under a low pressure of 0.1 mbar. [Japan. J. Appl. Physics 23(1984), pp. L268-L270]. The use of reactive silicon fluoride for the production of silicon nitride or silicon carbide powder or whiskers is, however, not previously known.